Exhaust purifier of an internal combustion engine

ABSTRACT

An exhaust purifier purifies an exhaust gas of an internal combustion engine used in an exhaust purification system that has an addition unit and a catalyst. The addition unit, disposed in an exhaust pipe of the exhaust purification system, adds an additive agent to the exhaust pipe, and the catalyst, disposed on a downstream side of the addition unit in the exhaust pipe, purifies the exhaust gas with the additive agent added by the addition unit and stored therein. The exhaust purifier determines a total amount of the additive agent to be added, and calculates an exhaust flow velocity of the exhaust gas. The purifier controls the addition unit to enable the additive agent to be added to the exhaust pipe by the total amount determined, such that the purifier controls the addition unit to decrease a per-unit-time addition amount of the additive agent as the exhaust flow velocity decreases.

CROSS REFERENCE TO RELATED APPLICATION

The present application is based on and claims the benefit of priority of Japanese Patent Application No. 2012-140010 filed on Jun. 21, 2012, the disclosure of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure generally relates to an exhaust purifier of an internal combustion engine in an exhaust gas purification system, which purifies a predetermined harmful ingredient by using a catalyst arranged in an exhaust pipe.

BACKGROUND

One type of conventional exhaust gas purification systems is known as a urea selective catalytic reduction (SCR) system. The urea SCR system uses a catalyst in an exhaust pipe, which selectively reduces and purifies NOx in an exhaust gas (i.e., an SCR catalyst, or a NOx selective reduction catalyst). An addition valve is disposed along an upstream side of the catalyst for spraying urea aqueous solution (i.e., an additive agent) into the exhaust pipe. The catalyst stores the urea aqueous solution that comes from the addition valve by converting the urea aqueous solution to ammonia, and resolves (i.e., chemically reduces) NOx in the exhaust gas to nitrogen and water by using the stored ammonia.

A storable amount of ammonia in the catalyst may change depending on a catalyst temperature. Therefore, it is necessary to optimize the stored amount of ammonia according to the catalyst temperature to increase the purification rate of NOx by using the catalyst. For example, if the stored amount of ammonia is too little, NOx reduction cannot be sufficiently performed. On the other hand, if the stored amount of ammonia exceeds the storable amount of the catalyst due to a steep change of the catalyst temperature or the like, an ammonia slip may be caused in which an excessive amount of ammonia at the current temperature is discharged from the catalyst. Such a drawback may be prevented by accurately estimating the stored amount of ammonia in the catalyst and by adequately adding the urea aqueous solution from the addition value based on the estimation of the stored amount of ammonia.

Conventionally, the stored amount of ammonia in the catalyst is estimated based on a balance among an exhaust amount of NOx from the internal combustion engine, an added amount of the urea aqueous solution from the addition valve (i.e., an NH3 amount), and an amount of ammonia that is consumed by the catalyst. Such a estimation technique is disclosed in Japanese patent No. 3,951,774 (U.S. Pat. No. 6,755,014 B2).

For an accurate estimation of the stored amount of the additive agent (i.e., ammonia) in the catalyst, it is necessary for the additive agent that is added from the addition valve to flow through the exhaust pipe to reach the catalyst with the exhaust gas. However, it is sometimes difficult for the additive agent to flow through the pipe without landing or condensing on a wall of the exhaust pipe, especially when the exhaust gas has a low energy flow at a low flow velocity, which prevents the additive agent to reach the catalyst. In such a case, the estimation of the stored amount of ammonia in the catalyst has some error, thereby leading to an addition of a not-so-adequate amount of the additive agent and deteriorating the purification rate by the catalyst.

SUMMARY

In an aspect of the present disclosure, an exhaust purifier purifies an exhaust gas of an internal combustion engine used in an exhaust purification system that has an addition unit and a catalyst. The addition unit, which is disposed in an exhaust pipe of the exhaust purification system, adds a predetermined additive agent to the exhaust pipe, and the catalyst, which is disposed on a downstream side of the addition unit in the exhaust pipe, purifies the exhaust gas with the additive agent added by the addition unit.

The exhaust purifier includes an addition amount determination unit, an addition control unit, and a flow velocity calculation unit. The addition amount determination unit determines a total amount of the additive agent to be added from the addition unit. The flow velocity calculation unit calculates an exhaust flow velocity of the exhaust gas.

The addition control unit controls the addition unit to enable the additive agent to be added to the exhaust pipe by the total amount determined by the addition amount determination unit. In particular, the addition control unit controls the addition unit to decrease a per-unit-time addition amount of the additive agent as the exhaust flow velocity calculated by the flow velocity calculation unit decreases.

According to the present disclosure, when the additive agent is added from the addition unit by an amount that is determined by the addition amount determination unit, the amount added per unit time is decreased according to the decrease of the exhaust flow velocity. When the exhaust flow velocity is low, an energy of the exhaust gas for conveying the additive agent to the catalyst becomes low. However, in such a case, the amount of the additive agent added per unit time is decreased. Therefore, the additive agent is efficiently mixed with the exhaust gas to achieve an efficient conveyance of the agent to the catalyst. In other words, the additive agent is prevented from landing or condensing on the wall of the exhaust pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

Other objects, features and advantages of the present disclosure will become more apparent from the following detailed description disposed with reference to the accompanying drawings, in which:

FIG. 1 is an illustration of an exhaust gas purification system of the present disclosure;

FIG. 2 is a flowchart of a process performed by an ECU of the exhaust gas purification system in a first embodiment;

FIG. 3 is a first example of a calculation method of a space velocity SV as an exhaust flow velocity;

FIG. 4 is a second example of a calculation method of the space velocity SV as the exhaust flow velocity;

FIG. 5 is a graph of a maximum NH3 storage amount against a catalyst temperature;

FIG. 6 is a graph of the maximum NH3 storage amount on one line and a target NH3 storage amount on another line against the catalyst temperature;

FIG. 7 is a map illustrating addition of urea aqueous solution by increasing a division number in a continuous manner according to a decrease of the exhaust flow velocity;

FIG. 8 is a map illustrating addition of urea aqueous solution by increasing the division number in a stepwise manner according to a decrease of the exhaust flow velocity;

FIG. 9 is a map illustrating addition of urea aqueous solution by selectively switching the division number at a threshold of the exhaust flow velocity;

FIGS. 10A and 10B are graphs of an added amount of urea aqueous solution and the NH3 storage amount in an SCR catalyst against the exhaust flow velocity of the first embodiment;

FIG. 11 is a flowchart of a process performed by the ECU in a second embodiment;

FIG. 12 is a map illustrating a driving duty for driving an addition value against the exhaust flow velocity of the second embodiment;

FIGS. 13A and 13B are graphs of the added amount of urea aqueous solution and the NH3 storage amount in the SCR catalyst against the exhaust flow velocity of the second embodiment;

FIG. 14 is a flowchart of a process performed by the ECU in a third embodiment;

FIG. 15 is a map illustrating a driving period against the exhaust flow velocity of the third embodiment; and

FIGS. 16 A and 16B are graphs of the added amount of urea aqueous solution and the NH3 storage amount in the SCR catalyst against the exhaust flow velocity of the third embodiment.

DETAILED DESCRIPTION First Embodiment

An exhaust purifier of an internal combustion engine regarding the present disclosure is described in the following with reference to the drawings. With reference to FIG. 1, an exhaust gas purification system 100 installed in a vehicle is configured as a urea SCR system for purifying a NOx containing exhaust that is discharged by a diesel engine 101, which is an internal combustion engine. The engine 101 has an exhaust pipe 13 connected thereto, and the exhaust gas from the engine 101 goes through the exhaust pipe 13 to be discharged outside of the vehicle.

The exhaust pipe 13 has an SCR catalyst 1 (i.e., a NOx selective reduction catalyst) disposed therein for selectively reducing NOx in the exhaust gas. The SCR catalyst 1 converts urea aqueous solution (i.e., an additive agent, or a reduction agent) added from a urea aqueous solution addition valve 2 into ammonia (NH3) by hydrolysis, and stores ammonia. The SCR catalyst 1 causes the following reactions represented by equation 1 or equation 2 between the stored ammonia and NOx in the exhaust gas, for the resolution (i.e., purification) of NOx to water and nitrogen. The urea aqueous solution addition valve 2 may be referred to as the addition valve 2 for brevity.

4NO+4NH3+O2→4N2+6H2O  (Equation 1)

6NO2+8NH3→7N2+12H2O  (Equation 2)

The storable amount of ammonia in the SCR catalyst 1 is not unlimited, and thus, has a certain limitation. With reference to FIG. 5, the maximum storable amount of ammonia in the SCR catalyst 1 changes depending on temperature of the SCR catalyst 1 (i.e., a catalyst temperature). Specifically, as the catalyst temperature increases, the maximum storable amount of ammonia (NH3) decreases.

The addition valve 2 adds or releases the urea aqueous solution in the exhaust pipe 13, and is disposed on an upstream side of the SCR catalyst 1 in the exhaust pipe 13. The addition valve 2 has a structure that is substantially the same as an injector (not illustrated) that injects fuel into a cylinder of the engine 101. In particular, the addition valve 2 is provided as an electro-magnetic valve that includes a drive unit, such as an electromagnetic solenoid, and a valve body that has a urea passage for passing the urea aqueous solution and a needle that opens-closes a tip jet nozzle, in which the valve opens-closes according to a drive signal from an ECU 11. Based on the drive signal, the electromagnetic solenoid receives electricity and the needle is moved to a valve open direction by such supply of electricity to open the nozzle, thereby adding (i.e., spraying or injecting) the urea aqueous solution from the tip jet nozzle.

The addition valve 2 is provided with a supply of the urea aqueous solution from a urea aqueous solution tank 7 as required. The configuration of a urea aqueous solution supply system is described in the following. For illustration purposes, assuming that the urea aqueous solution in the supply system flows from the tank 7 to the valve 2, a tank side (i.e., a position closer to the urea aqueous solution tank 7) is described as an upstream side of the urea aqueous solution supply system, and a valve side (i.e., a position closer to the addition valve 2) is described as a downstream side of the urea aqueous solution supply system.

The urea aqueous solution tank 7 includes an airtight container having a liquid supply cap, and stores therein a predetermined amount of the urea aqueous solution of predetermined density. The urea aqueous solution tank 7 and the addition valve 2 are connected with each other by a supply pipe 15, which has defined passage in which the urea aqueous solution travels. An inlet to suck in the urea aqueous solution is formed at a tip of the supply pipe 15 on the urea aqueous solution tank 7 side, and such inlet is immersed in the urea aqueous solution when the tank 7 is filled with the urea aqueous solution.

A pump 6 is provided along the supply pipe 15. The pump 6 is a line-type electric pump driven in a rotating manner according to a drive signal from the ECU 11. In the present embodiment, the pump 6 is disposed in the urea aqueous solution of the urea aqueous solution tank 7 in an immersed state. However, the pump 6 may be disposed outside of the urea aqueous solution tank 7.

A filter 3 having a porosity element is disposed along the supply pipe 15 for filtering the urea aqueous solution. Foreign matter is filtered by the filter 3, for protecting the valve 2 and the tank 7 from such foreign matter.

A pressure regulator 5 is provided on the downstream side of the pump 6, for adjusting a urea aqueous solution addition pressure to have a predetermined value. As a result of such adjustment by the regulator 5, a surplus of the urea aqueous solution is returned to the tank 7. Also, a pressure sensor 4, which is disposed along the supply pipe, detects a pressure of the urea aqueous solution in the supply pipe 15. Instead of mechanically controlling the urea aqueous solution addition pressure by the regulator 15, the urea aqueous solution addition pressure may be controlled through a drive control of the pump 6 based on the pressure detected by the sensor 4.

An exhaust flow sensor 9 for detecting an exhaust flow amount from the engine 101 is provided on an upstream side of the SCR catalyst 1 along the exhaust pipe 13. Specifically, the exhaust flow sensor 9 is provided on an upstream side of the addition valve 2.

Along the exhaust pipe 13 between the valve 2 and the catalyst 1 an exhaust temperature sensor 12 and an upstream side NOx sensor 81 are disposed. The exhaust temperature sensor 12 detects an exhaust temperature on the upstream side of the catalyst 1, and the upstream side NOx sensor 81 detects a density of NOx on the upstream side of the SCR catalyst 1. On a downstream side of the SCR catalyst 1 along the exhaust pipe 13, a downstream side NOx sensor 82 is disposed for detecting a density of NOx on the downstream side of the catalyst 1.

The exhaust gas purification system also includes an atmospheric pressure sensor 10 for detecting an atmospheric pressure. Each of the sensors 9, 12, 81, 82, 10 are connected to the ECU 11, and a detection value from each of the sensors 9, 12, 81, 82, 10 is inputted to the ECU 11.

The ECU 11 is equipped with a well-known microcomputer, for controlling an addition of the urea aqueous solution to the exhaust pipe 13 from the addition valve 2 based on the detection values of various sensors. Further, the ECU 11 includes a memory 111 to store various data (i.e., various maps mentioned later), which are used for a process performed by the ECU 11. The ECU 11 may serve as an “exhaust purifier of an internal combustion engine” in the present disclosure. Details of the process performed by the ECU 11 are described in the following with reference to FIG. 2. The process of FIG. 2 is started when, for example, the engine 101 is started, and is repeatedly performed at predetermined intervals until the engine 101 is stopped.

At S11, the ECU 11 acquires various data such as an exhaust temperature Tex (° C.), an atmospheric pressure Pa (Pa), and an exhaust flow amount Mf (kg/s) from the exhaust temperature sensor 12, the atmospheric pressure sensor 10, and the exhaust flow sensor 9, respectively. In the present embodiment, the exhaust flow amount Mf is provided as a mass of exhaust flow (kg/s). Alternatively, instead of using the detection value of the exhaust flow sensor 9, a detection value of an air flow meter, which detects an amount of an intake air taken into the engine 101, may be used as a replacement of the exhaust flow amount.

Subsequently, the ECU 11 calculates an exhaust flow velocity Vex, at S12. In the present embodiment, the ECU 11 calculates a space velocity SV of the exhaust gas in the SCR catalyst 1 as the exhaust flow velocity Vex.

To determine the space velocity SV, an exhaust density (kg/m3) of the gas on the upstream side of the SCR catalyst 1 is first calculated. The exhaust density may be calculated by, for example, substituting a temperature Tc (i.e., a catalyst temperature) of the SCR catalyst 1 and a pressure of the exhaust gas for variables of an equation of state of an ideal gas. Here, the catalyst temperature Tc is replaced with the exhaust temperature Tex. Further, the catalyst temperature Tc may be calculated based on the exhaust temperature Tex by preparing a map that represents a relationship between the catalyst temperature Tc and the exhaust temperature Tex.

By ignoring a pressure difference before and after the SCR catalyst 1, the atmospheric pressure Pa that is acquired at S11 is used as the pressure of the exhaust gas. After calculating the exhaust density, a volume flow amount (m3/s) of the exhaust gas is calculated by dividing the exhaust flow amount Mf (kg/s), which is received at S11, by the exhaust density (kg/m3). Subsequently, by dividing the volume flow amount (m3/s) by a volume (m3) of the SCR catalyst 1, the space velocity SV (1/s) is calculated. The volume of the SCR catalyst 1 is stored in the memory 111 in advance.

The space velocity SV is interrelated with the exhaust flow amount Mf, the atmospheric pressure Pa, and the catalyst temperature Tc (i.e., the exhaust temperature Tex). Therefore, at S12, the ECU 11 uses an SV calculation equation (i.e., SV (Mf, Pa, Tc)), as shown in FIG. 3, to calculate the space velocity SV by using the exhaust flow amount Mf, the atmospheric pressure Pa, and the catalyst temperature Tc, which are stored in the memory 111, as its parameters. Alternatively, as shown in FIG. 4, a map 201 that maps the space velocity SV against the catalyst temperature Tc, the exhaust flow amount Mf, and the atmospheric pressure Pa may be stored in the memory 111, and the space velocity SV may be calculated with reference to the map 201. The map 201 of the exhaust flow amount Mf and the atmospheric pressure Pa is prepared for different catalyst temperatures Tc.

With continuing reference to FIG. 2, the ECU 11 calculates a maximum NH3 storage amount stored in the catalyst at a current temperature Tc at S13. Specifically, with reference to FIG. 5, a map 203 representing a relationship between the catalyst temperature Tc and the maximum NH3 storage amount is stored in the memory 111. Based on the map 203, the ECU 11 calculates the maximum NH3 storage amount. Further, in this case, the exhaust temperature Tex, which is received at S11, is used in place of the catalyst temperature Tc, and the ECU 11 calculates the maximum NH3 storage amount STmax (Tex) for such exhaust temperature Tex.

Subsequently, the ECU 11 calculates a target storage amount of ammonia (i.e., a target NH3 storage amount) STtg (Tex) in the SCR catalyst 1 at S14. With reference to FIG. 6, a line 203 of the maximum NH3 storage amount at a certain catalyst temperature and a line 204 of the target NH3 storage amount are shown, where the line 203 is the same as the map 203 of FIG. 5. By designating the maximum NH3 storage amount at a certain catalyst temperature T1, as STmax(T1), the target NH3 storage amount may be set as STmax(T1) itself or an amount 202 that is close to STmax(T1). Based on such assumption, a situation of changing a driving state of the engine 101 from an idle state to an acceleration state is considered, in which the catalyst temperature (i.e., the exhaust temperature) steeply rises and the maximum NH3 storage amount steeply falls. As a result, excessive ammonia in the SCR catalyst 1 relative to the maximum NH3 storage amount of ammonia is discharged from the SCR catalyst 1 (i.e., an NH3 slip).

Therefore, in order to prevent the NH3 slip at a time of steep rise of the catalyst temperature, the ECU 11 sets the target NH3 storage amount to an amount having a certain margin to the maximum NH3 storage amount at S14. For example, the target NH3 storage amount is set at 60% to 70% level of the maximum NH3 storage amount. Further, the excessive amount of ammonia exceeding the maximum NH3 storage amount will not actually be discharged and wasted. That is, a part of the discharged amount of ammonia is consumed for the chemical reduction of NOx. The line 204 takes into account such a consumption of ammonia for setting the relationship between the catalyst temperature and the target NH3 storage amount. At S14, the ECU 11 may directly calculate the target NH3 storage amount from a map 204 (i.e., line 204) without going through the calculation of the maximum NH3 storage amount. In particular, the map 204 (i.e., the line 204), which represents a relationship between the catalyst temperature and the target NH3 storage amount, may be stored in the memory 111 in advance. In such a case, S13 of FIG. 2 may be omitted.

After S14, the ECU 11 calculates an amount of ammonia currently stored in the SCR catalyst 1 as an estimated NH3 storage amount STr at S15. To determine the estimated NH3 storage amount STr, the ECU 11 may first calculate a NOx discharge amount from the engine 101 based on a map that represents a relationship between (i) the NOx discharge amount and (ii) an engine rotation number and a fuel injection amount. A NOx purification rate of the SCR catalyst 1 is then determined based on the detection values from the NOx sensors 81, 82 provided along the upstream and downstream sides, respectively, of the SCR catalyst 1.

The ECU 11 further calculates a NOx amount purified of the SCR catalyst 1 (i.e., a NOx purification amount) based on the NOx discharge amount and the NOx purification rate. An ammonia consumption amount consumed by the SCR catalyst 1 is then determined based on the NOx purification amount. Since there is a correlation between the NOx purification amount and the ammonia consumption amount, the calculation of the ammonia consumption amount may be performed based on such correlation. In particular a map that represents the correlation may be stored in the memory 111 in advance. The ECU 11 calculates the estimated NH3 storage amount at a current time by subtracting the ammonia consumption amount from a pre-correction estimated NH3 storage amount. Alternatively, various methods including a method disclosed in Japanese patent No. 3,951,774 (patent document 1) may be employed as an estimation method for calculating the estimated NH3 storage amount STr.

The ECU calculates a deviation ΔST between the target NH3 storage amount STtg and the estimated NH3 storage amount STr at S16. In particular, the ECU 11 calculates STtg−STr=ΔST.

Based on the deviation ΔST calculated at S16, the ECU 11 calculates an added amount Qu (ΔST) of urea aqueous solution (i.e., a urea aqueous solution addition amount) that is injected from the addition valve 2, at S17. There are two kinds of chemical reduction of NOx: (i) a reduction by ammonia stored in the SCR catalyst 1 and (ii) a direct reduction directly performed by the urea aqueous solution (i.e., ammonia) from the addition valve 2. The ratio between the two kinds of chemical reduction may change depending on conditions such as the catalyst temperature. At S17, the ECU calculates the urea aqueous solution addition amount Qu by considering the amount of the urea aqueous solution that is consumed by the direct reduction (i.e., by considering the ratio of the two kinds of chemical reduction) for satisfying the target NH3 storage amount STtg. Specifically, the ECU 11 calculates the urea aqueous solution addition amount Qu by pre-storing an equation for calculating the urea aqueous solution addition amount Qu in the memory 111. The equation uses various conditions, such as the deviation ΔST and the catalyst temperature, as parameters. For instance, the urea aqueous solution addition amount Qu may be determined by the following equation: Qu=Qu1+(a*Qu2)+((1−a)*Qu3). Per the equation of “Qu”, “Qu1” is NH3 storage shortage amount and is based on the deviation ΔST; “Qu2” is an amount of NH3 that is directly consumed for chemical reduction and is based on NOx discharge amount; “Qu3” is an amount of NH3 that is consumed for the chemical reduction by the stored NH3 and is based on the NH3 storage amount and the catalyst's temperature; “a” is a ratio of direct chemical reduction and is based on the catalyst's temperature.

Based on the urea aqueous solution addition amount Qu calculated at S17, the ECU 11, at S18, determines a division number N, which represents the number of times the urea aqueous solution should be added from the addition valve 2. The division number N is a number used to divide the amount Qu into portions. In particular, when the exhaust flow velocity Vex (i.e., the space velocity SV) calculated at S12 is lower, the division number N should be greater. For instance, the division number N may be calculated based on a map 301 (FIG. 7), a map 302 (FIG. 8), or a map 303 (FIG. 9), which may be pre-stored in the memory 111.

In FIG. 7, when the exhaust flow velocity Vex is equal to or greater than a threshold Vth, the map 301 shows that the addition of the urea aqueous solution is performed only once by setting the division number N to 1 (i.e., the urea aqueous solution addition amount Qu is not divided into multiple portions). When the exhaust flow velocity Vex is less than the threshold Vth, the division number N continuously increases as the velocity Vex decreases. By calculating the division N according to the map 301, the addition of the urea aqueous solution is fine-tuned according to the exhaust flow velocity.

In FIG. 8, when the exhaust flow velocity Vex is equal to or greater than the threshold Vth, the map 302 shows that the addition of the urea aqueous solution is performed only once by setting the division number N to 1. When the exhaust flow velocity Vex is less than the threshold Vth, the division number N is increased in a stepwise manner as the exhaust flow velocity Vex decreases. In addition, as long as the urea aqueous solution is enabled to reach the SCR catalyst 1 without landing on a wall of the exhaust pipe 13, an interval ΔN between two adjacent division numbers N and an interval ΔVex between two exhaust flow velocities Vex may be arbitrarily set. By calculating the number N based on the map 302, a frequent change of the number N is prevented, which may otherwise be caused as a re-calculation of the number N whenever the exhaust flow velocity Vex changes.

In FIG. 9, the map 303 shows that the division number N is switched selectively for a high exhaust flow velocity and a low exhaust flow velocity. In particular, when the exhaust flow velocity Vex is equal to or greater than the threshold Vth, the map 303 sets the division number N to a value N1 (e.g., 1), and when the exhaust flow velocity Vex is less than the threshold Vth, the map 303 sets the division number N to a value N2 (i.e., N2>N1). By calculating the division number N based on the map 303, the calculation process of the division number N is simplified since the calculation is based only on a simple determination of the velocity Vex, which is either greater or smaller than the threshold Vth.

The ECU 11, at S19, then drives the addition valve 2 to add (i.e., release) the urea aqueous solution in the pipe 13 by a controlled amount that equal the urea aqueous solution addition amount Qu (i.e., a total amount in claims) that is calculated at S17. In such manner, the urea aqueous solution from the addition valve 2 is converted to ammonia by the SCR catalyst 1, and the ammonia is stored in the SCR catalyst 1. The amount of ammonia stored in the SCR catalyst 1 is then controlled to the target NH3 storage amount that is calculated in S14.

Further, at S19, when the division number N=1, the urea aqueous solution, which has the addition amount Qu, is added at one time. Alternatively, when the division number N is greater than or equal to 2, the amount of the urea aqueous solution added at each division time (i.e., each injection) is calculated by dividing the amount Qu equally by the division number N. Thus, the urea aqueous solution is added in equal amounts at each injection for N times. With reference to FIGS. 10A and 10B, the addition of the urea aqueous solution changes according to the exhaust flow velocity, which may be high or low, and the NH3 storage amount in the SCR catalyst 1 changes according to the change of the addition of the urea aqueous solution. FIG. 10A is a graph of a high exhaust flow velocity case, and FIG. 10B is a graph of a low exhaust flow velocity case. A line 21 provides a time change of the NH3 storage amount after the urea aqueous solution addition. Lines 22, 24 provide the time change of an actual NH3 storage amount (i.e., an estimated NH3 storage amount). A pulse line 23 and a pulse line 25 along the time t represent an interval at which the urea aqueous solution is added and the amount of the urea aqueous solution added at each interval. That is, the number of pulses in each of the pulse lines 23, 25 represents the number of times the urea aqueous solution is injected (i.e., division number N), where each pulse is an injection of the urea aqueous solution. Also, the size of each pulse of the pulse lines 23, 25 represents the amount of the urea aqueous solution added at each injection.

FIG. 10A illustrates an example of adding the urea aqueous solution at one time (i.e., N=1), and FIG. 10B illustrates an example of adding the urea aqueous solution by dividing the amount Qu into five portions (division number N=5), such that the urea aqueous solution is added five times at equal amounts.

When the exhaust flow velocity is high as shown in FIG. 10A, the division number N (i.e., 1 in FIG. 10A) is smaller than the division number N of the low exhaust flow velocity of FIG. 10B, which increases the amount of the urea aqueous solution added at each injection. Therefore, the estimated NH3 storage amount 22 is controlled to quickly (i.e., in a short time) approach the target NH3 storage amount 21. In contrast, as shown in FIG. 10B, when the exhaust flow velocity is low, the division number N (i.e., 5 in FIG. 10B) is greater than the division number N of the high exhaust flow velocity in FIG. 10A, which decreases the amount of the urea aqueous solution added at each injection. Therefore, the urea aqueous solution added is efficiently conveyed to the SCR catalyst 1 by preventing the urea aqueous solution from landing on the wall of the exhaust pipe 13. However, the time required for the estimated NH3 storage amount 24 to approach the target NH3 storage amount 21 in FIG. 10B is longer than the required time in FIG. 10A. When addition of the urea aqueous solution is performed at multiple times (FIG. 10B) an interval Δt between two injections at S19 is a constant value regardless of the division number N. After performing S19, the process of FIG. 2 is finished.

As described above, the addition of the urea aqueous solution in the present embodiment is performed so that the amount of added is controlled to approach the target NH3 storage amount that accords with the catalyst temperature. Therefore, even when the catalyst temperature changes, the deterioration of the NOx purification rate as well as the NH3 slip is prevented. Further, at the time the urea aqueous solution is added, when the exhaust flow velocity is low, the amount of the urea aqueous solution is decreased by increasing the number of times the urea aqueous solution is added (i.e., increasing the division number N), which prevents the urea aqueous solution from landing on the wall of the pipe. As a result, an estimation error in an estimation of the stored amount of ammonia in the SCR catalyst (i.e., an estimated NH3 storage amount) is decreased, which is beneficial for maintaining a high NOx purification rate.

Second Embodiment

The second embodiment of the exhaust purifier of the internal combustion engine regarding the present disclosure is described in the following, and focuses on differences from the first embodiment. In the second embodiment, an addition method of adding the urea aqueous solution from the urea aqueous solution addition valve is different from the first embodiment. Specifically, the addition valve is duty-controlled for adding the urea aqueous solution from the addition valve according to the driving duty.

The configuration of the exhaust gas purification system of the present embodiment is same as the configuration of the first embodiment in FIG. 1. FIG. 11 shows a flowchart of the process performed by the ECU 11. In FIG. 11, like numbers show like steps of the process of FIG. 2. In FIG. 11, S181 and S191 in are different from FIG. 2.

After S17, the ECU 11, at S181, calculates a driving duty D (i.e., a drive time τ against an addition period T: D=τ/T) when the process drives the addition valve 2. With reference to FIG. 12, the ECU 11 calculates the driving duty D based on a map 401, which is pre-stored in the memory 111. The map 401 defines a relationship between the exhaust flow velocity Vex and the driving duty D. As readily understood from FIG. 12, the map 401 defines a continuous decrease of the driving duty D when the exhaust flow velocity Vex decreases. Further, at S181, the driving duty D may be decreased in a stepwise manner as the exhaust flow velocity Vex decreases, similar to FIG. 8, or the driving duty D may be selectively switched between high and low for the high exhaust flow velocity Vex and for the low exhaust flow velocity Vex, similar to FIG. 9.

After S181, the ECU 11, at S191, controls the driving operation of the addition valve 2 so that the urea aqueous solution is added to the exhaust pipe 13 by the amount Qu calculated in S17. In particular, the ECU 11 provides the addition valve 2 with a driving pulse of the driving duty D that is calculated in S181. FIGS. 13A and 13B, which are similar graphs as the ones in FIGS. 10A and B, illustrate a time change of the NH3 storage amount after the addition of the urea aqueous solution. FIG. 13A is a graph of a high exhaust flow velocity case and FIG. 13B is a graph of a low exhaust flow velocity case. A pulse series 26, 27, represent a series of the driving pulses that are provided for the addition valve 2. A period T (i.e., an addition period) of the pulse series 26 of FIG. 13A is same as a period T of the pulse series 27 of FIG. 13B. A drive time τ2 of FIG. 13B is smaller than a drive time τ1 of FIG. 13A. In other words, a driving duty of FIG. 13B is smaller than a driving duty of FIG. 13A. Further, a total amount of the urea aqueous solution added from the addition valve 2 is the same for both cases, i.e., in FIG. 13A and FIG. 13B.

Therefore, when the exhaust flow velocity is high, as shown in FIG. 13A, the driving duty is high in comparison to the low exhaust flow velocity in FIG. 13B, thereby resulting in an increase of the amount added per unit time, which enables a quick approach of the estimated NH3 storage amount 22 to the target NH3 storage amount 21. In contrast, when the exhaust flow velocity is low as shown in FIG. 13B, the driving duty is low in comparison to the high exhaust flow velocity in FIG. 13A, thereby resulting in a decrease of the amount added per unit time and efficient addition of the urea aqueous solution that reaches the SCR catalyst 1 by preventing the urea aqueous solution from landing or condensing on the wall of the pipe 13. However, the time required for the estimated NH3 storage amount 24 to approach the target NH3 storage amount 21 in FIG. 13B is longer than the required time in FIG. 13A. After S191, the process of the flowchart in FIG. 11 is finished.

As described above, the present disclosure is applicable to a case where the addition valve is duty-controlled, for achieving the same effects as the first embodiment.

Third Embodiment

The third embodiment of the exhaust purifier of an internal combustion engine regarding the present disclosure is described in the following, and focuses on differences from the first and second embodiment. In the third embodiment, the addition method for adding the urea aqueous solution from the urea aqueous solution addition valve is different from the first and second embodiments, in which the addition valve is frequency-controlled for adding the urea aqueous solution periodically from the valve by driving the valve according to a driving period (i.e., at a driving frequency). In such frequency-controlled valve driving, an amount of the urea aqueous solution added at each drive is same, regardless of the driving period (i.e., the driving frequency).

The configuration of the exhaust gas purification system of the present embodiment is same as the configuration of the first embodiment as shown in FIG. 1. FIG. 14 shows a flowchart of the process performed by the ECU 11. In FIG. 14, like numbers show like steps of the process in FIG. 2. In FIG. 14, S182 and S192 in the process are different from the process of FIG. 2.

After S17, the ECU 11 calculates a driving period F at S182, when the ECU 11 drives the addition valve 2. More practically, the ECU 11 calculates a longer driving period F when the exhaust flow velocity Vex (i.e., the space velocity SV) calculated in S12 is lower. If the driving period F is converted (i.e., inverted) to a driving frequency (i.e., a reciprocal number of the driving period F), the lower the exhaust flow velocity Vex (i.e., the space velocity SV), the lower the calculated driving frequency should be.

More specifically, as shown in FIG. 15, the ECU 11 calculates the driving period F based on a map 501, which is pre-stored in the memory 111. The map 501 defines a relationship between the exhaust flow velocity Vex and the driving period F. The map 501 defines a continuous increase of the driving period F when the exhaust flow velocity Vex decreases. Further, at S182, the driving period may be increased in a stepwise manner as the exhaust flow velocity Vex decreases, similar to FIG. 8, or the driving period may be selectively switched between long and short for the high exhaust flow velocity Vex and for the low exhaust flow velocity Vex, similar to FIG. 9.

After S182, the ECU 11, at S192, controls the driving operation of the addition valve 2 so that the urea aqueous solution is added to the exhaust pipe 13 by the amount Qu calculated in S17. More practically, the ECU 11 provides the addition valve 2 with a driving pulse of the driving period F that is calculated at S182. FIGS. 16A and 16B, which are similar to FIGS. 10A and 10B, illustrate a time change of the NH3 storage amount after the addition of the urea aqueous solution. FIG. 16A is a graph of a high exhaust flow velocity case, and FIG. 16B is a graph of a low exhaust flow velocity case. A pulse series 28, 29 represent a series of driving pulses that are provided for the addition valve 2. Further, a pulse width of each of the driving pulses in the pulse series 28, 29 is the same (i.e., a constant pulse width). In other words, an amount of the urea aqueous solution added at each driving pulse is constant. Further, a driving period F2 in FIG. 16B is longer than a driving period F1 in FIG. 16A.

Therefore, when the exhaust flow velocity is high as shown in FIG. 16A, the driving period is shorter in comparison to the low exhaust flow velocity in FIG. 16B. Thus a quick approach of the estimated NH3 storage amount 22 to the target NH3 storage amount 21 is enabled. In contrast, when the exhaust flow velocity is low as shown in FIG. 16B, the driving period is longer in comparison to the high exhaust flow velocity in FIG. 16A, thereby resulting in a decrease of the amount added per unit time and efficient addition of the urea aqueous solution that reaches the SCR catalyst 1 by preventing the urea aqueous solution from landing or condensing on the wall of the exhaust pipe 13. However, the time required for the estimated NH3 storage amount 24 to approach the target NH3 storage amount 21 in FIG. 16B is longer than the required time in FIG. 16A. After S192, the process of the flowchart in FIG. 14 is finished.

As described above, the present disclosure is applicable to a case where the addition valve is frequency-controlled, for achieving the same effects as the first and/or second embodiments.

Also, in the first, second, and third embodiments the amount of the urea aqueous solution added per injection or per addition may be referred to as a “per-unit-time addition amount”.

Although the present disclosure has been fully described in connection with the preferred embodiment with reference to the accompanying drawings, it is to be noted that various changes and modifications will become apparent to those skilled in the art. For example, the exhaust purifier of the present disclosure is applicable to a system that uses both of the duty control in the second embodiment and the frequency control in the third embodiment. In such a case, for example, when the exhaust flow velocity is low, the driving duty is decreased and the driving period is increased for driving the addition valve in comparison to the high exhaust flow velocity case. Further, the present disclosure is applicable to the urea SCR system for a gasoline engine, or, more specifically, for a lean-burn gasoline engine. Further, the present disclosure is applicable to the exhaust gas purification system that uses a reduction agent other than the urea aqueous solution (e.g., a water solution containing ammonia).

Such changes and modifications are to be understood as being within the scope of the present disclosure as defined by the appended claims. 

What is claimed is:
 1. An exhaust purifier for purifying an exhaust gas of an internal combustion engine used in an exhaust purification system that has an addition unit and a catalyst, wherein the addition unit, disposed in an exhaust pipe of the exhaust purification system, adds a predetermined additive agent to the exhaust pipe, and the catalyst, disposed on a downstream side of the addition unit in the exhaust pipe, purifies the exhaust gas with the additive agent added by the addition unit and stored therein, the exhaust purifier comprising: an addition amount determination unit determining a total amount of the additive agent to add from the addition unit; an addition control unit controlling the addition unit to enable the additive agent to be added to the exhaust pipe by the total amount determined by the addition amount determination unit; and a flow velocity calculation unit calculating an exhaust flow velocity of the exhaust gas, wherein the addition control unit controls the addition unit to decrease a per-unit-time addition amount of the additive agent as the exhaust flow velocity calculated by the flow velocity calculation unit decreases.
 2. The exhaust purifier of claim 1, wherein the addition control unit adds the total amount of the additive agent in the exhaust pipe at one time.
 3. The exhaust purifier of claim 1, wherein the addition control unit adds the total amount of the additive agent in the exhaust pipe according to a division number that defines a number of times the additive agent is added, and the addition control unit increases the division number as the exhaust flow velocity decreases to lower the per-unit-time addition amount of the additive agent.
 4. The exhaust purifier of claim 1, wherein the addition control unit adds the additive agent to the exhaust pipe according to a duty-control of the addition unit, and the addition control unit decreases a driving duty as the exhaust flow velocity decreases to lower the per-unit-time addition amount of the additive agent.
 5. The exhaust purifier of claim 1, wherein the addition control unit adds the additive agent to the exhaust pipe according to a frequency-control of the addition unit, the frequency-control drives the addition unit periodically for a predetermined period, and the addition control unit increases the predetermined period as the exhaust flow velocity decreases to lower the per-unit-time addition amount of the additive agent.
 6. The exhaust purifier of claim 1, the addition amount determination unit further comprising: a temperature acquisition unit acquiring a temperature of the catalyst; a first calculation unit calculating a target storage amount indicating an amount of the additive agent stored in the catalyst as a temperature-dependent amount according to the temperature acquired by the temperature acquisition unit; a second calculation unit calculating an estimated storage amount indicating an amount of the additive agent currently stored in the catalyst; and a third calculation unit calculating a deviation between the target storage amount from the first calculation unit and the estimated storage amount from the second calculation unit, wherein the addition amount determination unit determines the total amount of the additive agent to be added to the catalyst based on the deviation from the third calculation unit.
 7. The exhaust purifier of claim 6, wherein the first calculation unit calculates the target storage amount as an amount that is smaller than a maximum storable amount of the additive agent in the catalyst at the acquired temperature.
 8. The exhaust purifier of claim 1, wherein the flow velocity calculation unit calculates the exhaust flow velocity as a space velocity of the exhaust gas in the catalyst.
 9. The exhaust purifier of claim 1, wherein the addition unit adds urea aqueous solution as the additive agent, the catalyst stores ammonia after converting the urea aqueous solution to ammonia, and is a NOx selective reduction catalyst that chemically reduces NOx in the exhaust gas by using the stored ammonia in the catalyst. 